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The disorders of xanthohistiocytic proliferation involving histiocytes, foam cells, and mixed inflammatory cells are divided into Langerhans cell histiocytosis (LCH) and non-Langerhans cell histiocytosis (non-LCH).
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LANGERHANS CELL HISTIOCYTOSIS
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LCH is an idiopathic spectrum of disorders characterized by a clonal proliferation of abnormal cells phenotypically similar to Langerhans cells of the skin. Clinically, LCH is characterized by lytic bony lesions and cutaneous findings that range from soft tissue swelling to eczema- and seborrheic dermatitis-like skin changes and ulceration.
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INSIGHT 
Histiocytosis can be extremely difficult to diagnose. In infants with diaper rash that will not heal, particularly if there are erosions in the folds or petechiae/purpura, histiocytosis should be considered.
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SYNONYMS Class I histiocytosis, nonlipid reticuloendotheliosis, eosinophilic granulomatosis. Previously termed Histiocytosis X.
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In recent years, a concerted effort has been made to aggregate subforms of the disease under the single disease term LCH, given overlap in clinical and pathophysiologic feature. Prior classification approaches distinguished between the following overlapping subtypes, which we present for historical reference:
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Letterer–Siwe disease: an aggressive form of LCH with diffuse skin and organ infiltration and thrombocytopenia
Hand–Schüller–Christian disease: LCH with lytic skull lesions, exophthalmos, and diabetes insipidus
Eosinophilic granuloma: single osteolytic bony lesion ± skin/soft tissue lesion
Hashimoto–Pritzker disease: congenital self-healing reticulohistiocytosis.
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Given the rarity of the disease, we encourage the single term LCH to encompass the spectrum of disease.
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AGE Any age, most common 1 to 3 years.
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INCIDENCE Rare, 3 to 5/million children.
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GENETICS Familial case reports. Mutations in the BRAF oncogene have been seen in a large percentage of LCH lesions.
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The proliferating Langerhans-like cell appears to be primarily responsible for the clinical manifestation of LCH. The stimulus for the proliferation may be a disturbance of intracellular lipid metabolism, a reactive response to infection (possible viral), a primary immunologic disorder of the host, or an inherited neoplastic disorder. Gene expression studies have demonstrated upregulation of factors associated with T-cell recruitment and stimulation in LCH, as well as growth factors including TGF-β.
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LCH has a broad clinical spectrum, but in the most aggressive form, the infant appears systemically ill with a generalized skin eruption (seborrhea, petechiae, and purpura) followed by fever, anemia, thrombocytopenia, adenopathy, hepatosplenomegaly, and/or skeletal lesions, demonstrating multisystem involvement. Conversely, in cases of single-system LCH, the affected individual may be asymptomatic.
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TYPE Papules, plaques, vesicles, scale, petechiae, purpura, ulceration, necrosis.
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COLOR Pink, flesh-colored.
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SIZE 1 to 2 mm up to 1 cm in ...